In Situ Microstructure Observation Of Steel Grades In The Semi-Solid State For Thixoforging Process By Using Confoncal Laser Scanning Microscopy

Abstract

The microstructure plays a crucial role for steel semi-solid forming process, and particularly for the steel thixoforging process, since it determines the thixotropic flow behavior of materials in the semi-solid state. Therefore, it is necessary to well understand the microstructure evolution during high speed heating and forming. Classically, it is investigated on a solid material quenched from semi-solid state by 2D characterization techniques. However, the semi-solid microstructure could probably not be preserved in the solid state by quenching due to complicated phase transformations or high diffusion rate of alloying elements during cooling, especially at low liquid fractions. In order to avoid this, a new in situ technique - high temperature Confocal Laser Scanning Microscopy (CLSM) - was developed and used for studying the microstructure evolution directly at high temperature. The present study aims at providing an experimental investigation of the microstructure evolution on several steel grades (M2, 100Cr6 and C38LTT) during heating from the as-received state to the semi-solid state (heating rate: ~200°C/min) and finally cooled to the solid state (cooling rate: ~200°C/min). It has been found that the temperature sensitivity of liquid fraction (ΔT/ Δfl) of these grades is much different. In addition, during cooling, there is a significant difference in diffusion rate of alloying elements between these grades. In M2, thanks to the high content of alloying elements and their low diffusion rate, the semi-solid temperature range is greater and its microstructure in the semi-solid state could be preserved by quenching process or even at a low cooling rate. On the contrary, the microstructure of other steel grades 100Cr6 and C38LTT in semi-solid state can only be revealed by CLSM at high temperature, because of the lower volume fraction of alloying elements and their high diffusion rate. It is very interesting to use high temperature CLSM to in situ investigate the microstructure evolution in the semi-solid state, especially at low liquid fraction. Since the microstructure of M2 in the semi-solid state can be reserved after cooling, it is possible to identify liquid area by the post-mortem examination. Therefore, the material flow behavior of thixoforged parts could be investigated.